1. Field of the Invention
The present invention relates to a control method of a scanner optical system of an original image reading apparatus, and in detail, to a method of return control, that is, control of returning a scanner to a reference position after original image reading scanning has been finished. Further, the present invention relates to a motor control device and a carriage driving device of an image reading apparatus.
2. Description of the Related Art
FIGS. 1A and 1B show a general arrangement in one example of an original image reading apparatus to which the present invention is applied. FIG. 1A shows a general side elevational view and FIG. 1B shows a general perspective view. As it is well known, an exposing optical system including a scanner optical system 2 is provided below a contact glass 1 on which an original image sheet is placed. Thereby, reflected light from the original image sheet is used for forming an image on a drum-shaped light sensitive body 3. The scanner optical system 2 includes a first scanner 7 including a light source 4, a reflecting plate 5, a first mirror 6 and a second scanner 10 including second and third mirrors 8 and 9. The exposing optical system is formed by the scanner optical system 2, an image forming lens 11 fixed to the apparatus body, a fourth mirror 12 and so forth. A dustproof glass 13 is provided. The first scanner 7 and the second scanner 10 are driven by a driving motor 14 via a wire 15. In the apparatus body, a scanner home position sensor (hereinafter referred to as `HP sensor`) is provided. The HP sensor 16 acts as reference position detecting means of the scanner optical system 2 and is of a reflection photosensor. A covering plate 17 for covering the HP sensor 16 is provided to the first scanner 7.
In the above-described arrangement, the first and second scanners 7 and 10 of the scanner optical system 2 start from a home position shown by solid lines in FIG. 1A and are driven rightwardly so as to expose and scan a surface of the original image sheet. After the scanner optical system 2 has finished the exposing and scanning, the first and second scanner 7 and 10 are at positions shown by chain double-dashed lines in FIG. 1A. This position varies depending on a size of an original image sheet. Return control is performed on the scanner optical system 2 which has finished the exposing and scanning so that the scanner optical system 2 is returned to the home position for a subsequent exposing and scanning.
In such an original image reading apparatus performing going and returning operations, when performing the going operation for exposing and scanning an original image sheet, fine velocity control is performed for preventing degradation of reading image quality. However, when performing the returning operation for returning the scanner optical system to the reference position (home position) for a subsequent original image exposing and scanning, the return control is performed wherein, in order to reduce the scanner moving time, the scanner optical system is driven with full power and braking is performed at a predetermined position, for example.
For example, in the art disclosed in Japanese Laid-Open Patent Application 2-232641, acceleration is performed using a predetermined returning velocity as a target velocity. Then, after reaching a predetermined position, deceleration is performed using a returning velocity obtained from a predetermined calculation equation or data table as a target velocity. An image scanning system is driven at various scanning velocities for various scanning distances depending on the size of an original image sheet or the size of a copy sheet. Accordingly, a plurality of calculation formulas and data tables are prepared and appropriate ones are selected therefrom according to a particular situation. Further, when the velocity of the scanner optical system varies with respect to the target velocity, the predetermined position at which velocity control is changed to deceleration control is appropriately changed. Specifically, the moving velocity and the position (moving distance) of the reading unit are detected, errors between the target velocity and detected velocity are calculated and a proportional integral control operation is performed.
However, in the above-described methods, in order to reduce the returning time, the scanner optical system is accelerated to high velocity. Thereby, when the scanner optical system is decelerated, appropriate deceleration may not be performed. As a result, the scanner optical system may not be stopped at the home position, and vibration and noise may occur.
Japanese Laid-Open Patent Application No. 5-22975 discloses a motor drive control method and apparatus. In the method and apparatus, when performing motor deceleration control, target angular velocity .omega. of a motor in deceleration is obtained as follows: EQU .omega.=.sqroot.2.alpha..THETA.
where .THETA. represents the angle which the motor rotates until the motor stops, and .alpha. represents the absolute value of an angular acceleration at the deceleration. Thus, the motor is decelerated at the fixed angular acceleration.
Although it is not disclosed in Japanese Laid-Open Patent Application No. 5-22975, in order to perform high-speed control, a high-voltage (for example, 38 V) power source is needed, and, in order to perform stable control, ordinarily current control is performed. Accordingly, a current sensor is needed specially for this purpose. As a result, the control system is expensive.
Further, in the art of Japanese Laid-Open Patent Application No. 5-22975, when performing motor deceleration control, target angular velocity .omega. of a motor in deceleration is obtained as follows: EQU .omega.=.sqroot.2.alpha..THETA.
where .THETA. represents the angle which the motor rotates until the motor stops, and .alpha. represents the absolute value of an angular acceleration at the deceleration. Thus, the motor is decelerated at the fixed angular acceleration. However, there is no disclosure of a method of determining the angular acceleration at the deceleration.